Researchers from the Harvard School of Public Health found the risk of autism rises in parallel with exposure to fine particulate matter during pregnancy, with the biggest effect occurring in the final months of gestation. The results appear in the Dec. 18 edition of Environmental Health Perspectives.

The findings add to other research suggesting the environment plays a role in the development of autism, a developmental disorder marked by repetitive behaviors and trouble communicating and socializing. The study, which started in 1989 and involved more than 100,000 nurses from across the U.S., will help researchers home in on the causes of autism and potential ways to prevent it, said Marc Weisskopf, a senior study author.

“One of the unique aspects of the study we did is that it provides an even stronger piece of evidence for there being a causal effect,” said Weisskopf, an associate professor of environmental and occupational epidemiology at Harvard in Boston. “It’s really the pollution doing it.”

Autism, thought to affect 1 in 68 children in the U.S., is typically diagnosed after behavioral changes start to develop before the age of 5. Recent studies suggest it may begin when certain brain cells fail to properly mature within the womb.

Study’s Scope

Researchers focused on 1,767 children born from 1990 to 2002, including 245 diagnosed with autism. The design of the study and the results rule out many confounding measures that can create a bias, Weisskopf said. The researchers took into account socioeconomic factors that can influence exposure to pollution or play a role in whether a child is diagnosed with autism.

The fact that pollution caused problems only during pregnancy strengthened the findings, since it’s unlikely other factors would have changed markedly before or after those nine months, he said in a telephone interview.

The ultimate cause of autism remains a mystery in most cases, said Charis Eng, chairwoman of the Lerner Research Institute’s Genomic Medicine Institute at the Cleveland Clinic in Ohio. While the Harvard study isn’t definitive and the findings could be coincidental, it’s not likely given the large size and the precise results, she said in a telephone interview.

“The truth is there has to be gene and environmental interactions,” said Eng, who wasn’t involved in the study. “I suspect the fetus already had the weak autism spectrum disorder genes, and then the genes and the environment interacted.”

If the child didn’t have the genetic predisposition, the impact may have been minimal or nonexistent, she said.

Body’s Response

It’s likely there is an inflammatory or immune system response to the pollution that reaches the fetus, Weisskopf said. His team is now exploring those biological pathways and mapping autism cases to see if there are any clusters. He emphasized that many things contribute to the disorder and the absolute risk from pollution may be very small.

Fine particulate matter stems from many different sources, including traffic and power plants located hundreds of miles away. There is no way to avoid it entirely, though pregnant women may want to try to curtail their exposure when possible, Weisskopf said. He recommended against trips to cities with high levels of pollution and exercise in traffic-clogged areas during pregnancy.